PERF: Cache Category.subcategory_ids (#9350)

Also reset category cache after backup restore.

app/models/category.rb

@@ -74,6 +74,7 @@ class Category < ActiveRecord::Base
   after_save :publish_discourse_stylesheet
   after_save :publish_category
   after_save :reset_topic_ids_cache
+  after_save :clear_subcategory_ids
   after_save :clear_url_cache
   after_save :index_search
   after_save :update_reviewables
@@ -153,23 +154,36 @@ class Category < ActiveRecord::Base
     Category.reset_topic_ids_cache
   end
 
+  @@subcategory_ids = DistributedCache.new('subcategory_ids')
+
   def self.subcategory_ids(category_id)
-    sql = <<~SQL
-        WITH RECURSIVE subcategories AS (
-            SELECT :category_id id, 1 depth
-            UNION
-            SELECT categories.id, (subcategories.depth + 1) depth
-            FROM categories
-            JOIN subcategories ON subcategories.id = categories.parent_category_id
-            WHERE subcategories.depth < :max_category_nesting
+    @@subcategory_ids[category_id] ||=
+      begin
+        sql = <<~SQL
+            WITH RECURSIVE subcategories AS (
+                SELECT :category_id id, 1 depth
+                UNION
+                SELECT categories.id, (subcategories.depth + 1) depth
+                FROM categories
+                JOIN subcategories ON subcategories.id = categories.parent_category_id
+                WHERE subcategories.depth < :max_category_nesting
+            )
+            SELECT id FROM subcategories
+          SQL
+        DB.query_single(
+          sql,
+          category_id: category_id,
+          max_category_nesting: SiteSetting.max_category_nesting
         )
-        SELECT id FROM subcategories
-      SQL
-    DB.query_single(
-      sql,
-      category_id: category_id.to_i,
-      max_category_nesting: SiteSetting.max_category_nesting
-    )
+      end
+  end
+
+  def self.clear_subcategory_ids
+    @@subcategory_ids.clear
+  end
+
+  def clear_subcategory_ids
+    Category.clear_subcategory_ids
   end
 
   def self.scoped_to_permissions(guardian, permission_types)

app/models/topic.rb

@@ -191,9 +191,7 @@ class Topic < ActiveRecord::Base
   }
 
   scope :in_category_and_subcategories, lambda { |category_id|
-    where("topics.category_id = ? OR topics.category_id IN (SELECT id FROM categories WHERE categories.parent_category_id = ?)",
-        category_id,
-        category_id) if category_id
+    where("topics.category_id IN (?)", Category.subcategory_ids(category_id.to_i)) if category_id
   }
 
   scope :with_subtype, ->(subtype) { where('topics.subtype = ?', subtype) }
@@ -1296,7 +1294,7 @@ class Topic < ActiveRecord::Base
     builder = DB.build(sql)
     builder.where("t.created_at >= :start_date", start_date: opts[:start_date]) if opts[:start_date]
     builder.where("t.created_at < :end_date", end_date: opts[:end_date]) if opts[:end_date]
-    builder.where("t.category_id IN (:subcategory_ids)", subcategory_ids: Category.subcategory_ids(opts[:category_id])) if opts[:category_id]
+    builder.where("t.category_id IN (:subcategory_ids)", subcategory_ids: Category.subcategory_ids(opts[:category_id].to_i)) if opts[:category_id]
     builder.where("t.archetype <> '#{Archetype.private_message}'")
     builder.where("t.deleted_at IS NULL")
     builder.where("p.deleted_at IS NULL")
@@ -1335,7 +1333,7 @@ class Topic < ActiveRecord::Base
     builder = DB.build(WITH_NO_RESPONSE_SQL)
     builder.where("t.created_at >= :start_date", start_date: start_date) if start_date
     builder.where("t.created_at < :end_date", end_date: end_date) if end_date
-    builder.where("t.category_id IN (:subcategory_ids)", subcategory_ids: Category.subcategory_ids(category_id)) if category_id
+    builder.where("t.category_id IN (:subcategory_ids)", subcategory_ids: Category.subcategory_ids(category_id.to_i)) if category_id
     builder.where("t.archetype <> '#{Archetype.private_message}'")
     builder.where("t.deleted_at IS NULL")
     builder.query_hash
@@ -1355,7 +1353,7 @@ class Topic < ActiveRecord::Base
 
   def self.with_no_response_total(opts = {})
     builder = DB.build(WITH_NO_RESPONSE_TOTAL_SQL)
-    builder.where("t.category_id IN (:subcategory_ids)", subcategory_ids: Category.subcategory_ids(opts[:category_id])) if opts[:category_id]
+    builder.where("t.category_id IN (:subcategory_ids)", subcategory_ids: Category.subcategory_ids(opts[:category_id].to_i)) if opts[:category_id]
     builder.where("t.archetype <> '#{Archetype.private_message}'")
     builder.where("t.deleted_at IS NULL")
     builder.query_single.first.to_i

config/initializers/014-track-setting-changes.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 DiscourseEvent.on(:site_setting_changed) do |name, old_value, new_value|
+  Category.clear_subcategory_ids if name === :max_category_nesting
+
   # Enabling `must_approve_users` on an existing site is odd, so we assume that the
   # existing users are approved.
   if name == :must_approve_users && new_value == true

lib/backup_restore/restorer.rb

@@ -52,6 +52,7 @@ module BackupRestore
 
       @system.disable_readonly_mode
 
+      clear_category_cache
       clear_emoji_cache
       clear_theme_cache
 
@@ -119,6 +120,12 @@ module BackupRestore
       end
     end
 
+    def clear_category_cache
+      log "Clearing category cache..."
+      Category.reset_topic_ids_cache
+      Category.clear_subcategory_ids
+    end
+
     def clear_emoji_cache
       log "Clearing emoji cache..."
       Emoji.clear_cache

lib/topic_query.rb

@@ -669,7 +669,7 @@ class TopicQuery
       if options[:no_subcategories]
         result = result.where('categories.id = ?', category_id)
       else
-        result = result.where(<<~SQL, subcategory_ids: subcategory_ids(category_id), category_id: category_id)
+        result = result.where(<<~SQL, subcategory_ids: Category.subcategory_ids(category_id), category_id: category_id)
           categories.id in (:subcategory_ids) AND (
             categories.topic_id <> topics.id OR categories.id = :category_id
           )
@@ -1052,11 +1052,6 @@ class TopicQuery
 
   private
 
-  def subcategory_ids(category_id)
-    @subcategory_ids ||= {}
-    @subcategory_ids[category_id] ||= Category.subcategory_ids(category_id)
-  end
-
   def sanitize_sql_array(input)
     ActiveRecord::Base.public_send(:sanitize_sql_array, input.join(','))
   end